1. Field of the Invention
The present invention relates to a piezoelectric thin film component and the manufacturing method thereof. The present invention also relates to an inkjet-recording head using the piezoelectric component, and an inkjet printer using this head. The present invention more particularly relates to a new improvement of a piezoelectric thin film component where residual strain is minimized.
2. Description of the Related Art
An actuator using a piezoelectric thin film component, which converts electric energy into mechanical energy or vice versa, is used for a pressure sensor, a temperature sensor, an inkjet type recording head, and for other purposes. In the inkjet type recording head, a piezoelectric thin film component is used as an actuator to be the drive source of ink ejection.
This piezoelectric thin film component generally comprises a piezoelectric thin film comprised of polycrystalline substances, and a top electrode and bottom electrode which are disposed sandwiching the piezoelectric thin film. The composition of the piezoelectric thin film is generally a binary system in which the main component is lead zirconate titanate (hereafter "PZT"), or a ternary system where a third component is added to the binary system.
The piezoelectric thin films with such composition are generated, for example, by a sputtering method, sol-gel method, MOD process (Metal Organic Decomposition process), laser ablation method and CVD method. For example, a ferro-electric substance using the binary system PZT is noted in "Applied Physics Letters, 1991, Vol. 58, No. 11, pp. 1161.about.1163". Also, piezoelectric materials using the binary system PZT is disclosed in Japanese Patent Laid-Open No. 6-40035, and in the "Journal of the American Ceramic Society, 1973, Vol. 56, No. 2, pp. 91.about.96".
When a piezoelectric thin film component is applied to an inkjet type recording head, for example, a piezoelectric thin film (PZT film) with a 0.4 .mu.m.about.20 .mu.m film thickness is appropriate. The piezoelectric thin film needs a high piezoelectric charge constant, therefore it is normally necessary to perform heat treatment at a 700.degree. C. or higher temperature to grow crystal grains of the piezoelectric thin film.
When a piezoelectric thin film (PZT film) with a 0.5 .mu.m or higher film thickness is formed, performing heat treatment to obtain a high piezoelectric charge constant causes cracks inside the film, which is a problem.
A method disclosed in "Philips J. Res. 47 (1993) pp. 263.about.285" is creating a sol or gel composition, baking at high temperature to crystallize the piezoelectric thin film, and repeating this process to increase the film thickness of the piezoelectric thin film.
The piezoelectric thin film created by this method, which has a multi-layered interface, cannot present good piezoelectric characteristics, and processability is poor. Repeating the heat treatment also leads to deterioration of piezoelectric characteristics, such as crystals losing orientation.
A piezoelectric thin film is normally formed on a bottom electrode, which is formed on a substrate, and heat treatment performed to form the piezoelectric thin film causes curvature and strain on the substrate, which is a problem. Also good adhesion is required between the bottom electrode and the piezoelectric thin film.
So the present inventor and others considered various ways to increase the piezoelectric charge constant of a piezoelectric thin film, and discovered that it is effective if the crystals of the piezoelectric thin film have a predetermined crystal orientation and columnar structure, and also have a crystal structure with a grain size of 0.1 .mu.m to 0.5 .mu.m (Japanese Patent Application No. 9-288757).
However, the present inventor and others further examined and discovered the following problem. When an electric field is applied to a virgin state piezoelectric thin film component, residual strain and polarization strain remain in the piezoelectric thin film component, even after the electric field is removed, and good piezoelectric strain characteristics (displacement characteristics) cannot be obtained. In other words, if an electric field is applied to a piezoelectric thin film and this is polarized, the domain (crystal grains) of the piezoelectric materials creating the piezoelectric thin film moves such that the polarization axis matches with the direction of the electric field. Then cavities are generated in the grain boundaries of the grains, which seems to be causing residual strain.